Phase-locked loop demodulators that are capable of demodulating low IF signals are presently available. These devices convert the RF carrier to the I and Q quadrature components at low IF which are processed in a phase-locked loop. An example of such a demodulator is described in U.S. Pat. No. 4,852,123 which issued on Jul. 25, 1989 to Bickley et al.
These demodulators are generally unstable as the IF nears zero-IF. In addition, these circuits which include loop filters as well as a voltage controlled oscillator with further filters and mixers to drive the phase-locked loop, are quite complicated.
Therefore, there is a need for a simpler coherent FM demodulator in low-IF or zero-IF for radio receiver architectures. In addition, it is desirable to coherently demodulate FM signals with extremely low modulation indexes and significant centre frequency offset even when such offsets are several times greater than the maximum FM modulation deviation.